Stainless steel is used in a large number of applications and industries, including such wearable, ornamental or portable items as watch cases, watch bracelets, bracelets, eye-glass frames, jewelry items, and mobile phones. For aesthetic reasons, various surface finishes, such as polished, satin, and sand-blasted, are employed.
Stainless steel is usually of a hardness of 200 HV under the Vicker scale. As a result of this relatively low degree of hardness, stainless steel surfaces are easily scratched or otherwise damaged in daily use. The appearance and attractiveness of the aforesaid items, in particular watch cases and watch bracelets, will be blemished, thus affecting the value of such products.
Various methods have thus been devised for treating stainless steel to increase its hardness, including, e.g. nitridation and carburization. In nitridation, the surface of the stainless steel is hardened by formation of nitrides. Nitrogen is introduced into the stainless steel usually by heating the stainless steel in gaseous ammonia. In carburization, the outer layer of low-carbon stainless steel is converted into high-carbon stainless steel by contact with a carbonaceous material. Both nitridation and carburization can considerably increase the surface hardness of stainless steel. By carburization, the surface hardness of the treated stainless steel can reach around 800 HV; whereas by nitridation, a surface hardness of around 600 HV can be reached. Both processes, however, require high temperature treatments for a long period of time. For example, nitridation has to be carried out at a temperature of 524–549° C., and takes from 24 to 48 hours to complete the process. For carburization, the temperature is even higher, to the degree of 1,000° C., and takes about 20 hours to complete the process.
Both nitridation and carburization are based on the principle of solid diffusion, in which atoms of impurities, such as nitrogen and carbon, diffuse from the stainless steel surface into the interior of the bulk substrate. The diffusion concentration profile is gradual, and the depth of diffusion is usually between 20–30 microns. Improved hardness of the treated stainless steel is brought about by the formation of chromium nitride in the case of nitrided stainless steel, and chromium carbide in the case of carburized stainless steel. The degree of hardness is the highest at the surface of the treated stainless steel, where the concentration of the chromium nitride or chromium carbide is the highest, and decreases as the depth into the interior of the bulk substrate increases.
As discussed above, long treatment time and high treatment temperature are required for nitridation and carburization, thus decreasing throughput and production efficiency, and increasing the production cost. In addition, nitridation and carburization processes are also associated with the following drawbacks. Firstly, hardness and wear-resistance of the treated stainless steel are obtained at the expense of corrosion resistance. As there is no fresh supply of chromium, the formation of chromium nitride or chromium carbide will deplete the chromium content in the original stainless steel grain, thus adversely affecting the corrosion resistance capability of the treated stainless steel.
Secondly, nitridation and carburization can take place only on austenite stainless steel substrates, which are with high original chromium concentration, but not on martensite stainless steel substrates.
Thirdly, in order to enhance the efficiency of diffusion, the surface concentration of either nitrogen or carbon in the treated stainless steel is rather high, thus darkening or blemishing the surface of the treated stainless steel. Post-diffusion surface treatment, e.g. polishing, is thus required to return as much as possible the surface of the treated stainless steel to the original colour. Such treatment is usually performed manually, thus lacking control and consistency.
Another method of hardening stainless steel surface is by electroplating thereon relatively thick, e.g. over 10 microns, layers of chromium or tungsten. However, the surface colour of the so-treated stainless steel will also be very different from that of usual stainless steel. The hardness of the electroplated stainless steel is also lower than that of nitrided or carburized stainless steel. Carbides of chromium and tungsten can also be deposited on stainless steel substrates by physical vapor deposition, resulting in high hardness of the surface of the treated substrates. However, again, the surface colour of the so-treated stainless steel differs appreciably from that of usual stainless steel, thus hindering the mass acceptance of products made of such treated stainless steel in the market.
In addition, for such metals and metal alloys as copper, copper alloys, aluminum, aluminum alloys, magnesium, magnesium alloys, titanium, and titanium alloys, as there is little or no chromium, neither nitridation nor carburization of substrates made of such material is possible. Metallic hard chromium electroplating is only possible for application on copper and its alloys. Aluminum and its alloys, magnesium and its alloys, and titanium and its alloys oxidize rapidly in electrolytic solutions, and become electrically un-conductive, thus preventing electroplating. Although coating of various carbides and nitrides of high hardness can be deposited on these substances by physical vapor deposition (PVD), the appearance of such carbide or nitride coatings are very different from stainless steel. Furthermore, PVD of ceramics such as carbide or nitride uses radio frequency (RF) power supplies, e.g. at a frequency of 13.56 MHz, and the low deposition rates add much to the manufacturing cost.
It is thus an object of the present invention to provide a method of depositing a nanocomposite coating onto a solid metal or metal alloy substrate to increase the surface hardness of the substrate, in which the shortcomings associated with the conventional methods discussed above are mitigated, or at least to provide a useful alternative to the public.
It is also an object of the present invention to provide an article deposited with a nanocomposite coating, according to a method disclosed herein.